Instagram, the popular photo sharing app on the Apple iPhone, has reached 27 million users and is expected to launch on Google Android "very soon".

Speaking at the South by Southwest event this weekend, Instagram founder Kevin Systrom told TechCrunch that the app is the "single fastest-growing thing" in mobile.

Instagram is a free photo sharing app that allows users to apply digital filters to their photos, and then share them with others via social networking tools, including Instagram's own system.

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Systrom said that Instagram has reached 27 million registered users worldwide since it launched on Apple's iOS in 2010.

Discussing Instagram's users, he said: "They're not excited about it because it makes your photos look beautiful. They're excited because it networks people across the world and it's the single fastest-growing thing in mobile period."

The Android version of Instagram is currently being tested in private beta, and it is reportedly even better and "faster" than the iOS version.

Co-founder Mike Krieger said that the Android team have been impressed with the way the app leverages the operating system. He also defended the decision to focus first on iOS rather than launching on both platforms at the same time.

"I don't think it took us so long. We just had priorities. Had we tried to be both on Android and iPhone at the same time, it would've been tough to innovate in the way that we have," he said.

Instagram does not currently carry any advertising or have in-app purchases like other social networks. But Krieger was coy on how the company would make money in the future.

"We have a visual platform and advertisers like visual mediums. They like TV and magazines, but attention is moving online and they want to switch," he said.

"I do believe that Instagram has put a stake in the ground and we're growing more quickly than anyone. Is there something in there we could do to make it a multi-billion dollar business? I think we can figure out something along the way."